Radiation area detector systems have become a valuable tool for experimentation and research in a wide variety of scientific and medical applications. Over the past few years, these detectors have become increasingly important as analytical and diagnostic devices which are used in such diverse fields as crystallography, medical radiography, electron microscopy, biophysics, and even astronomy. Previously, these area detection devices generally fell into either of two distinct types: (1) the multi-wire proportional counter, such as described in Bateman et al, Nuc. Inst. Meth. Res. A259: 506-520 (1987), and (2) the T.V. detector such as disclosed in Kalata, Methods in Enzymology 114: 486-510 (1985). These two types of area detection devices are still used today in various applications.
These previous devices, however, have suffered from several drawbacks. In particular, they are often extremely limited in terms of active area and spatial resolution, and often experience high levels of spatial distortion and non-uniformity of response. The devices also generally require a prolonged exposure to X-rays in order to develop a satisfactory picture. In cases where an instantaneous image of a rapidly deteriorating sample is required, these prior systems have not been rapid enough to provide near real time images, and thus are not suitable for such an application.
In the field of protein crystallography, instantaneous imaging is almost always necessary and a device that can provide near real time images is often required. During crystal growth, protein single crystals are grown so that the three-dimensional structure of the protein can then be determined by X-ray or other radiation diffraction patterns. Typically, these grown crystals deteriorate very rapidly with increasing time and handling, and the specific details of the protein structure will often be lost if an X-ray pattern from the crystals cannot be obtained within an extremely short period after their formation. It is thus extremely important to develop systems for area detection which have rapid data acquisition and which can provide near real time imaging capabilities for X-ray diffraction patterns.
A relatively recent discovery of the unique properties of certain phosphor-containing films has enabled new developments in X-ray and ultraviolet-sensitive area detection devices. In particular, it has been found that a plate containing a barium fluorohalide (such as BaFX:Eu) crystal will absorb a particular fraction of incident X-ray or UV radiation by "trapping" an electron in a halogen ion vacancy or "F-center". Electrons so trapped will normally be stored at a half life of approximately 10 hours. However, if the film containing the trapped electrons is irradiated with visible light, the electrons in the F-center will be liberated to the conduction band which leads to the formation of Eu.sup.+2 ions in an excited state. These excited ions then relax to give off luminescence in proportion to the intensity of the X-ray or UV radiation originally absorbed by the film. It is thus possible through the use of such film to create a stored or latent image on the film which can almost instantaneously be dumped or otherwise transmitted to an image translation means by subsequent illumination of the film by an appropriate wavelength of light or other electromagnetic wave. Further, after the dumping of the image, the phosphor film will return to its original state so as to be reusable for further X-ray imaging.
Devices incorporating such a phosphor-containing film are also known in the art. An example of one is found in Miyahara et al, Nuc. Inst. Meth. Phys. Res. A246: 572-578 (1986), and this device essentially consists of a barium fluorohalide phosphor screen imaging plate, a laser beam reflected by a scanning mirror, a light guide for collection of the photostimulated luminescent radiation, and a tube for collecting the fluoresced light. In this device, a He--Ne laser beam emitting light at about 633 nanometers is reflected by the mirror and used to illuminate the film which luminesces at around 400 nm in response to the laser stimulation.
Another area detection device is also disclosed in U.S. Pat. No. 4,933,558 (Carter et al), incorporated herein by reference, wherein a light source either directly or through a mirror illuminates the phosphor-containing film storing the X-ray image, and the fluoresced light is focused upon a charged coupled detecting element by means of a lens or an optical fiber bundle in the shape of a half-hour glass. Both of these devices can be used to scan the film line-by-line, but are limited in that the pixel rsesolution obtained in the line scan cannot be easily changed.
There are still other problems with prior art side-by-side scanning devices which have restricted their effectiveness. One such problem is that they tend to create skewed-shaped pixels on the outer edges, and there often are problems relating to spontaneous fluorescence which contributes to unwanted background noise. In addition, it is prefered that the means to illuminate the phosphor-containing film be physically as close to the film as possible, yet this is hard to accomplish using any of these prior devices. It is also desirable to accomplish injection and detection of the excitation and fluoresced radiation along the same optical path in order to maximize the efficient transmission of light and ensure a high spatial resolution in the scanning device.
It is thus a desirable object to develop an X-ray and UV or radiation sensitive area device of high spatial resolution that can utilize a phosphor-containing film in a manner which allows for line-by-line scanning with easy manipulation of pixel resolution, injection and detection of excitation and fluorescence using the same optical pathway, and the placement of the illuminating light source as physically close to the phosphor film as possible. A device utilizing these features can be used successfully to produce near real time images of high spatial resolution from rapidly deteriorating samples such as would be involved in research in the field of protein crystallography.